With the liquid-phase synthesis routes for nanoparticles of controlled compositions and uniform particle sizes well established, attention has now turned towards control of nanoparticle shape. Up to now tetrapod-shaped crystals with dimensions on the nanometer and micrometer scale have been synthesized for a variety of II-VI semiconductors including ZnO, CdS, CdTe, and CdSe. CdTe tetrapod-shaped nanocrystals have previously been synthesized using CdO as the cadmium precursor in oleic acid (OA)-trioctylphosphine (TOP) system or in a mixture of surfactant of octadecylphosphinic acid (ODPA) and the trioctylphosphine oxide (TOPO)-TOP system, respectively.
The hot injection route—in which molecular precursors are introduced rapidly into a hot ligand-containing organic solvent—was pioneered by Murray et al. (1993 J Am Chem. Soc. 115:8706) for the synthesis of photoluminescent CdSe and other metal non-oxide nanoparticles, collectively called quantum dots (sometimes referred to as “QDs”). Quantum dots with rod, rice-like, tetrapod, branched, hollow-shell, and other shapes can be synthesized, though high shape selectivity and size uniformity remain difficult to achieve for nanoparticles with more complicated shapes and compositions.
We note that nearly all hot-injection methods for synthesizing non-spherical quantum dots involve alkylphosphonates (e.g., hexylphosphonics, tetradecylphosphonic, and octadecylphosphonic acids) as a second ligand at high concentrations. They are thought to bind to certain quantum dot faces preferentially and sufficiently to modify growth kinetics, unlike other electron-rich ligands (i.e., phosphine oxide, phosphine, amine, and carboxylate head groups).
Unfortunately, hot injection routes using alkylphosponates do not produce CdSe tetrapods with high selectivity and the phosphonic acid ligands that are used by these methods are exhorbitantly expensive. A method that provides a more cost effective route to nanoscale particles would be useful. A method that also provides improved shape-selectivity, particularly for CdSe tetrapods, would also be useful.